I want to compare two types of engines: Turboprops and piston engines. I want to know which one is more efficient. There are many types of efficiencies, so I will specify: thrust (in newtons) per fuel consumption (kg/second).

Right now I don't care about power/weight ratio or max speed or horsepower. Those are all valid concerns for aircraft design, but I'm trying to pick exactly one criteria and figure which engine is more efficient in its terms and its terms alone. I don't want to mix criteria. I want to focus on one aspect, get an answer to it, then move on to the others later.

A complication is that piston engines usually give their output in horsepower. I have no idea why this is. What's ultimately needed is thrust. I realize that a propeller engine's job is to create torque, and thrust will depend on blade length and airspeed. In that case, I can't understand why those engines don't specify their output in torque.

Citing engines that actually exist (and citing their specs) will answer this question as long as the choices are modern, good engines.

(I did not include turbojets or turbofans for fear of being too broad. I picked two propeller engines and don't want to mix them with jet engines.)

One more thing. I realize that engines are capable of varying levels of thrust. It's my understanding that max thrust is usually more inefficient than cruising thrust. Therefore, I will ask for thrust/fuel-consumption figures of an engine at typical cruise conditions.

4 Answers
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Their output is given in kilowatts or HP because this does not change much with speed, unlike the thrust of a propeller. By running a piston engine a few times on a dynamometer stand you can get reasonable numbers which are valid over the full operating range. If you want to characterize them by their thrust, you would need to look at an engine-propeller combination at one particular speed, which is not very helpful.

Now you ask for specs of contemporary engines. The funny thing is that the efficiency of aviation piston engines has not changed much over the last decades. If you assume 250 g of fuel per kWh at full power for gasoline engines, this figure holds already for good WW II-era engines like the Jumo 213 A which was run on 87 octane gasoline. The low octane number restricted compression to 6.93:1, while the higher octane number of contemporary AVGAS allows for an 8.5:1 compression ratio in engines like the Lycoming O-360, which consumes 280 g of fuel per kWh. Adding fuel injection enabled Lycoming to reduce fuel burn to 240 g per kWh in the IO-390, which was first run in 2002.

Diesel engines are even more efficient; they typically consume 220 g per kWh. This low value was already possible with the venerable Jumo 205 of the 1930s which consumed only 213 g per kWh at its most efficient speed. Modern aerodiesels run at similar efficiency: The Thielert range of engines which have been taken over by Continental consume 220 g per kWh.

Even the best turboprops rarely achieve less than 300 g per kWh. The most modern version of the venerable Pratt&Whitney PT6 consumes 308 g per kWh, and only very recent developments can close the gap to piston engines. The Progress D27 claims a specific fuel consumption of 231 g per kWh while the Europrop TP400 one of 237 g per kWh. Note that here the remaining thrust from the exhaust has been converted into an equivalent power rating to achieve such good values.

$\begingroup$The higher efficiency of diesel engines is illusory, and is an artifact of measuring fuel by volume instead of mass. Diesel fuel has a higher energy volume (joules/litre) than gas, by about the proportion you quote. This also shows up "at the pump" with diesel priced about the same proportion above gas. See also the way copper wire has half, or double, the conductivity of aluminum wire, depending on whether you compare by mass or volume.$\endgroup$
– HarperMay 13 '18 at 16:30

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$\begingroup$@Harper: Yes, Diesel fuel has a higher specific mass, but you forget the higher compression ratio of Diesels which makes them inherently more efficient. Poor individual designs are no excuse.$\endgroup$
– Peter KämpfMay 13 '18 at 16:58

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$\begingroup$@Harper: also notice that the answer is specifically about g/kWh instead of liters/kWh.$\endgroup$
– Mikko RantalainenJan 27 at 12:21

There is no small turbine engine that has anywhere near the specific fuel consumption (Lbs/Hp?Hr or Grams/Kw/Hr) of a gasoline or diesel aircraft engine. Turbines are light (lbs/Hp or KG/Kw) but burn considerably more fuel. Turbines are also extremely smooth, almost no vibration. But as to efficiency, even our antique technology small aviation piston engines, turbines just aren't very good.

Typically the smaller the turbine the worse the specific fuel consumption. So the Allison, now Rolls, C250 family, are terrible, sometimes getting 0.8 Lbs/Hp/Hr in low cruise setting. The big turbines, like the AE2100, can get down to 0.4, but these engines are rated at 5,000hp, much too big for light aircraft.

One particularly troublesome aspect of turbine specific fuel consumption is that it is worse at lower power settings. This is one of the reasons that most turbine aircraft tend to fly as high as possible, because at high altitude they are running much closer to max power to achieve best aerodynamic efficiency. Piston engines tend to achieve their best specific fuel consumption at relatively low power settings, so flying low and slow does not hurt range nearly as much as on a turbine aircraft.

With several new diesel aircraft engines coming into use the efficiency of piston aircraft engines has gotten much better. Some of these diesels have specific fuel consumption in the 0.35 lb/Hp/Hr range, better than even the most efficient large aircraft turbines, and close to half of a small turboprop at low cruise.

Turbines aren't as efficient as piston mills, but the difference isn't as much as you might think if you consider the improved performance. At optimum altitude, the Meridian [turboprop] burns about 31 gph compared to 20 gph on the Mirage [piston], roughly 50% more. That's because piston engines are more efficient and offer a lower specific fuel consumption (.43 lbs./hp/hr) compared to turbines (.58 lbs./shp/hr).

$\begingroup$It has an obvious error though: "Jet fuel is 13% heavier than avgas, so the Meridian must carry more fuel..." Heavier fuel is good, it means more fuel can be carried in the same tanks. Engines consume a quantity (mass) of fuel, not volume (which their own figures quoted above correctly show).$\endgroup$
– ZeusJun 27 '16 at 9:15

If you want to rate an engine by the thrust it makes, you must know the propeller efficiency at the given criteria for your test. The engine performance and thrust performance both change with altitude and speed. Only a jet engine or turbofan engine should be rated by thrust, and even then, it changes with altitude and speed. Don't accept book numbers for prop efficiency, because it changes all over the map.